1. Field of the Invention
The present invention relates to a process for connection between an optical fiber and an optical microguide. The optical fiber can be a monomode or monomodal fiber or a multimode or multimodal fiber
2. Discussion of the Background
The optical microguide is of the type which comprises, on a substrate, a guidance structure incorporating a core called the "microguide core" and located between two media, whose respective refractive indices are below the refractive index of the microguide core.
By definition, a guidance structure is a structure resulting from the superimposing of three media, the intermediate medium (the core) having a refractive index higher than that of the two other media.
A microguide is a special guidance structure, whereof at least one of the three media constituting the guidance structure is laterally limited (e.g. by etching), so as to ensure a lateral confinement of the light.
The axis of the microguide core (corresponding to the microguide axis) is then defined as an axis located mid-height of the medium forming the core, parallel to the axis of the laterally limited or defined medium or media and contained in the same plane as said axis, said plane being perpendicular to the planes of the layers of the guidance structure.
Strictly speaking the core axis should be defined from the overlap integral of the guided modes of the fiber and the microguide, which must be at a maximum. In practice, said axis is always very close to the geometrical axis of the previously defined core. Therefore it is the latter axis which will be considered throughout the remainder of the description for simplicity reasons.
Moreover, for greater clarity, the following description refers to a microguide, whereof only the medium forming the core is laterally limited, it being obvious that the invention applies to all types of microguides.
On returning to the two media between which is located the microguide core, the upper medium, i.e. that not in contact with the substrate, can be air, but in general the adjacent media to the core are constituted by two layers of appropriate materials or the same appropriate material, which can be silica when the substrate is of silicon.
The present invention more particularly applies to the field of integrated optics. In this field, the coupling operation between a monomode or multimode optical fiber and an optical microguide is important and must be at the same time:
(1) effective, i.e. must lead to connection losses not exceeding approximately 0.5 to 1 dB, PA0 (2) fast and therefore, if possible, must not require micropositioning of the fiber before the latter is rendered integral with the optical microguide and PA0 (3) inexpensive, while integrating in the best possible way with the other technical operations leading to the production of the optical microguide.
The present invention relates to a process able to satisfy the aforementioned requirements.
FIGS. 1 and 2 illustrate the importance of obtaining a correct connection between an optical fiber and an optical microguide. In an exemplified manner use is made of a fiber of type STL CW 1505 E, whose core has a diameter of 8 micrometres and which is used with a light, whose wavelength is 1.55 micrometers.
FIG. 1 shows the variations of the coupling efficiency R as a function of the misalignment of the optical axis of the fiber and the optical axis of the microguide (said misalignment being expressed in micrometers), for a microguide having, between two silica layers, a doped silica core, whose cross-section is shaped like a square with a 4.8 micrometer side length and whose optical index exceeds that of the adjacent layers by a value equal to 6.multidot.10.sup.-3 (i.e., curve I in FIG. 1) or 7.multidot.10.sup.-3 (i.e., curve II in FIG. 1).
FIG. 2 shows the variations of the coupling efficiency R as a function of the distance (in micrometers) between the aforementioned optical fiber and an optical microguide having, between two silica layers, a doped silica core, whose optical index exceeds by 7.multidot.10.sup.-3 that of the adjacent layers and whose cross-section is shaped like a square with a 5 micrometre side length (i.e., curve I of FIG. 2) or the shape of a rectangle 6 micrometers long and 5 micrometers wide (i.e., curve II of FIG. 2).
Connection processes between an optical fiber and an optical microguide are already known.
One of these known processes is diagrammatically illustrated in FIG. 3 and firstly consists of forming in a support 2, a V-shaped recess 4 for receiving the optical fiber 6. When the latter is put in place in its recess 4, the microguide 8 is positioned on the support 2 facing the fiber.
This microguide comprises, e.g. on a substrate 10, two silica layers between which is located the microguide core and the positioning is carried out in such a way that the substrate 10 is not in contact with the support 2, so that it is one of the two silica layers which is in contact with said support.
The depth of the V-shaped recess 4 is such that the axis of the core 12 of the fiber 6, which is put into place in said recess 4, projects beyond the surface of the support 2 and substantially coincides with the axis of the optical microguide core 14.
The relative lateral positioning of the optical fiber 6 with respect to the optical microguide 8 generally takes place with the aid of not shown rails formed both on the support 2 and at least in the silica layer in contact with the support 2.
This known process suffers from the disadvantage of requiring two supports, namely the support 2 and the substrate 10 of the optical microguide 8. Moreover, bearing in mind the alignment constraints, it is very difficult to obtain an effective coupling between the optical fiber and the optical microguide without dynamic alignment of the two supports (i.e. without micropositioning of the microguide 8 and therefore its substrate 10 with respect to the support 2), which leads to a long connection time and to high connection costs.
According to a second known process diagrammatically illustrated by FIG. 4, the optical fiber 6 is put into place in a V-shaped recess 16 formed in the support 18, like support 2, said support 18 being put into place on another support 20 and, in turn, the microguide 8 is placed on the support 20, so that its substrate 10 is in contact with the support 20. The depth of the recess 16 is such that the optical fiber core axis can then be substantially aligned with the axis of the core of the microguide 8.
Therefore this second known process is even more complicated and costly than the first known process, because it requires an additional support.